1. Field of the Invention
The present invention concerns a magnetic resonance system of the type having a base unit and a patient bed, wherein the base unit embodies a magnet system with which magnetic fields are generated in an excitation region, that cause nuclear spins in an examination subject moved into the excitation region to be excited so as to emit a magnetic resonance signal, and wherein the patient bed can be moved in a travel direction relative to the base unit through a travel range, such that the examination subject on the patient bed can be brought into the excitation region by movement of the patient bed.
2. Description of the Prior Art and Related Subject Matter
Magnetic resonance systems of the above type are generally known.
Conventionally, the acquisition of the magnetic resonance signals emitted from the examination subject ensues either with a whole-body antenna or with local coils. If the magnetic resonance signal is acquired with a whole-body antenna, the magnetic resonance signal can be acquired from the entire excitation region. The acquisition is possible, only with relatively low spatial resolution and with a relatively low signal-to-noise ratio (SNR). Therefore in many cases local coils are also used in magnetic resonance systems, often even a number of local coils. The local coils are arranged near to the examination subject (normally a person) and therefore can receive with good SNR, if only from a small portion of the excitation region. Due to the spatial resolution, the spatial coding with gradient fields can be expanded by the arrangement of the local coils. The measurement time for an acquisition (scan) can thus also be reduced.
In order to be able to utilize the advantages of local coils for imaging of the entire body of a person from head to foot, a large number of local coils is required that are placed on the patient in a number of planes arranged along the travel direction of the patient bed. These planes are often designated as levels.
The excitation region typically exhibits a length of approximately 40 to 60 cm as viewed in the travel direction. Thus, only a portion of the body of the person can be imaged at a time, namely the portion that is located in the excitation region. For this reason the patient bed with the patient located thereupon must be moved incrementally through the excitation region. The local coils located in the excitation region must respectively be activated and be connected with an evaluation device for evaluation of magnetic resonance signals. The other local coils can be deactivated. It is thereby possible to manage with a relatively low number of reception channels even though many local coils are present.
In magnetic resonance systems of the prior art the aforementioned problem is achieved by all local coils being connected by a corresponding number of plugs attached at the patient bed and a movable cable harness to an evaluation device that is arranged at the base body. Due to the attenuation of the long, thin cables that are used, pre-amplifiers must be arranged in the local coils. Each local coil must furthermore have a detuning circuit in order to be able to deactivated given non-usage and given transmission. Complicated common mode chokes (known as sheath wave barriers) must also be inserted into the long cable harness in order to be able to limit induced voltages in transmission.
For the back region of the patient it is possible to arrange the acquisition array in a fixed manner under the movable patient bed in the excitation region. In this case the number of these local coils must only be sufficient for the excitation region. Thus for imaging the back region of the patient, it is possible to save many local coils as well as, their cabling and channel selection. The distance from the examination subject is increased only by the relatively slight thickness of the patient bed itself, which is for the most part quite tolerable.
By contrast, this procedure cannot be realized, or can just barely be realized, on the top side of the examination subject because the thicknesses of the patient and the various body regions of the patient are very different. Primarily for thin patients or, for example, at the head or legs, an acquisition coil array permanently installed in the excitation region would be far removed from the body surface, such that the advantage of the local coils (namely a high spatial resolution and a good SNR) would be lost.
A plug connection for local coils that operates without contact (namely by inductive coupling) is already known from DE 101 30 615 C2. This teaching already represents an advance since a galvanic contact between the local coil and the evaluation device is no longer required for coupling the local coil to the evaluation device. However, as before the requirement of an active plugging of the connection by operating personnel exists. The local coil must also be manually connected to the evaluation device or separated from it.
From DE 35 00 456 C2 it is known to couple a local coil with the whole-body antenna. Here a contactless coupling is in fact realized, but, this coupling is only possible for a single coil, and even that only given a suitable orientation of the local coil. The teaching of DE 35 00 456 C2 thus cannot be extended to a number of local coils. Here as well the local coil must be actively connected to the whole-body antenna or be separated from it.
From EP 0 437 049 A2 it is known to directly, inductively couple one local coil to another coil that is arranged in the immediate proximity of the local coil. In this arrangement as well the local coil must be actively connected to the evaluation device or disconnected from it.
A magnetic resonance system of the aforementioned type is likewise described in German patent application 10 2005 056 711.8. In this magnetic resonance system a patient bed coupling element that is connected with a first local coil for acquisition of the magnetic resonance signal is arranged on the patient bed so as to be stationary relative to the patient bed. A number of base unit coupling elements that are connected with an evaluation device for evaluation of magnetic resonance signals are connected at the base unit, and are stationary relative to the base unit. The base unit coupling elements and the patient bed coupling element are arranged and fashioned such that the magnetic resonance signal acquired by the local coil can be fed via the patient bed coupling element and the base unit coupling elements to the evaluation device when and as long as the patient bed is moved by a predetermined segment of the travel range.
The German patent application 10 2005 056 711.8 has not been published as of the priority date of the present invention, and therefore does not represent prior art.
The magnetic resonance system described in the German patent application 10 2005 056 711.8 operates quite well, and allows a local coil to be automatically coupled to an evaluation device when it is located in the excitation region, and is otherwise forcibly decoupled from the evaluation device.
In the embodiment of the German patent application 10 2005 056 711.8, the degree of coupling with which the patient bed coupling element and the base unit coupling element are coupled with one another overall varies with the travel path by which the patient bed is moved within the segment of the travel range.